The bonding wire used as a mounting material of a semiconductor chip connects a semiconductor chip and external metal terminals. At the present time, almost all such wire is made using a material mainly comprised of gold. The major reason for this is that semiconductor chips and external terminals are connected using the high throughput and high productivity technique known as “ball bonding”.
“Ball bonding” is the method of bonding melting one end of a metal wire to form a ball, pressing this against one electrode to bond with it, and press bonding the side surface of the wire to another electrode as it is.
Here, the ball bonding will be referred to as the “first bonding” and the bonding for press bonding the side of the wire to an electrode will be referred to as “second bonding”.
The reason why gold is used so much is that there is little deterioration of the first and second bondabilities due to surface oxidation of the balls or wire and bonding in the atmosphere is easy.
The bonding wire is increased in strength by work hardening by wire drawing, but with pure gold, a sufficient mechanical strength is not obtained, so trace amounts of different elements are added. However, the generally used additive elements easily oxidize at the time of forming the balls, so if added in large amounts, they degrade the bondability.
Therefore, it is being sought to obtain a high strength without adding these elements as much as possible. In the second bonding as well, the effect of the additive elements is felt.
In general, the less the additive elements, the less the surface oxidation of the wire, so the better the second bondability obtained.
In recent years, the mounting sizes of semiconductor chips have become smaller, the sizes of electrode pads have become smaller, and the pitches have become narrower. Along with this, the bonding wire has also been reduced in thickness. Gold wire of a diameter of 15 μm is also starting to be used.
If the wire size becomes smaller, in wire production, the wire will not be strong enough and will break at the time of drawing and maintenance of the loop at the time of bonding, resin sealing, and other mounting will become difficult, so the gold wire has to be increased in strength.
As additive elements for raising the gold wire strength, calcium and rare earth elements (lanthanides including scandium and yttrium) may be mentioned. Numerous patents have been disclosed relating to additive elements (for example, Japanese Patent Publication (A) No. 58-96741, Japanese Patent Publication (A) No. 58-154242, Japanese Patent Publication (A) No. 4-284821, Japanese Patent No. 2621288, Japanese Patent No. 2661247, Japanese Patent No. 2680414, Japanese Patent No. 2766706, Japanese Patent No. 2773202, Japanese Patent No. 2814660, Japanese Patent No. 3337049).
However, among these, the rate of contribution to the strength is not clear. For example, Japanese Patent Publication (A) No. 58-96741 reports the strength of materials by lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, and yttrium alone or by composite addition, but no difference of these elements with respect to strength can be recognized.
Further, Japanese Patent Publication (A) No. 58-154242 discloses that cerium family rare earth elements of lanthanum, cerium, praseodymium, neodymium, and samarium improve the ordinary temperature and high temperature tensile strength, but their actions are equal.
On the other hand, Japanese Patent No. 3337049 describes that among the rare earth elements, europium is superior to calcium and other rare earth elements in reliability of the ball neck. Here, praseodymium is an additional ingredient. When not contained together with any of platinum, silver, magnesium, and europium, no effect is recognized in the reliability of the ball neck.
The contribution to strength differs even among the rare earth elements, so in terms of raising the strength, there are optimal combinations. There can be said to be additive elements suitable for this, but as explained above, there are various reports on the contribution of additive elements to strength and the situation is not clear.